September 13, 2019

Tropical storm warnings are up in the northwestern Bahamas ahead of "Potential Tropical Cyclone Nine," a tropical disturbance that will likely develop into Tropical Storm Humberto over the next day or two. Its ultimate strength and track remains to be seen, but conditions are favorable for strengthening and the potential storm could bring foul weather to areas devastated by Hurricane Dorian last week.

Don't focus too much on the name of the storm right now. "Potential Tropical Cyclone" is a bureaucratic thing that lets forecasters issue watches and warnings before a disturbance becomes a tropical cyclone. The system is likely going to develop into a tropical depression on Friday and a tropical storm by this weekend.

A tropical storm warning is in effect for the northwestern Bahamas, including Great Abaco and Grand Bahama, the two islands devastated by category five Hurricane Dorian last week. Gusty winds and 2-4 inches of rain are possible in the northern Bahamas as the future tropical storm moves through the area. Tropical storm force winds (39+ MPH) are possible in the vulnerable northwestern Bahamas as early as Friday morning.

The official forecast from the National Hurricane Center brings future-Humberto into Florida as a tropical storm. The main impacts right now look to be gusty winds—some strong enough to down trees and power lines—and heavy rain, which could lead to flooding issues across areas that saw heavy rain from Dorian last week. Based on the current forecast track, the Weather Prediction Center expects a broad potential for 3-6 inches of rain across coastal parts of the southeast, with higher totals in some areas. Rainfall forecasts will likely change as forecasters get a better handle on the organization and ultimate track of the storm.

There's considerable uncertainty in forecast track for this potential storm. It's important to remember that this system isn't a fully-formed tropical cyclone. There's no center of circulation at the surface and the system hasn't developed the core that's necessary for a tropical cyclone to sustain itself and grow. The organization of a system heavily influences its future track; a weak and unstable system is more susceptible to sudden shifts, while a stronger and more well-rooted system can tap into deeper atmospheric steering currents.

We'll know much more about the future of this system once it actually develops. I know it seems like a million years ago, but we had a similar problem during the early days of Dorian. It wasn't a sure thing that the storm would survive beyond the Greater Antilles because of the young system's weak and fragile structure. Future-Humberto is a different system and it's rooted in a much different environment than the one that supported Dorian. That said, the ex-storm's first days in existence are a great example of how the internal structure of a storm can have a big influence on what it does in the future.

Now, about that name...

A "potential tropical cyclone" is a disturbance that's on the cusp of developing into a tropical depression or tropical storm, but it's close enough to land that watches and warnings are needed to give people adequate time to prepare for tropical storm conditions.

The old rules used to forbid the NHC from issuing watches or warnings until the system actually developed into a tropical depression or tropical storm. Sometimes it's too late to issue warnings by the time the storm actually develops.

One infamous example of this was Tropical Storm Bill back in 2015. Bill made landfall in Texas with 60 MPH winds just 12 hours after a tropical storm warning went into effect. If you went to bed early on June 15 and woke up late on June 16, you could have missed the entire warning ahead of the storm. That issue is remedied by the existence of "potential tropical cyclones."

Thankfully, the NHC usually doesn't have to use this designation for very long. These systems are so close to the line that they're typically upgraded to a depression or a storm within a couple of advisories, limiting the time that this newfangled term is thrown around in weather forecasts.

I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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